1. Technical Field
This disclosure relates generally to an electric rotating machine which is to be mounted in automotive vehicles such as passenger vehicles or trucks.
2. Background Art
Typical electric generator or alternators mounted in automotive vehicles work to supply charging power or operating power to a storage battery or various types of electric loads through a charging wire coupled to an output terminal thereof. Accidental removal of the output terminal or disconnection of a battery terminal from the battery during a power generating mode of the alternator will result in a surge of voltage called load dump. The peak of such surge may be as high as 100V or more depending upon the degree of output current from the alternator, which will be a factor resulting in damage to the electric loads or electric components of the alternator. It is, thus, necessary to take measures against such a voltage surge. For example, Japanese Patent First Publication No. 2012-16158, as assigned to the same assignee as that of this application, teaches an automotive alternator equipped with a load dump protector. Specifically, the alternator includes a bridge circuit whose low-side switching device is a MOS transistor and, when an output voltage from the alternator which has arisen from the load dump has exceeded a reference voltage, turns on the MOS transistor at a time when a voltage spike which arises from the turning on of the MOS transistor is expected not to be developed, thereby eliminating the risk of the voltage surge.
The disconnection of one of ends of the charging wire from the output terminal of the alternator or of the other end of the charging wire from the battery terminal are thought of as the cause for the load dump. When the charging wire is disconnected from the battery terminal, a rise in output voltage from the alternator which results from the load dump will be relatively small because of connection of other electric loads to the charging wire or impedance of the charging wire itself. The automotive alternator, as taught in the above publication, waits for an opportunity when the voltage spike will not occur and then turns on the MOS transistor, thereby avoiding application of the voltage spike to the electric loads connected to the charging wire.
Alternatively, when the charging wire is disconnected from the output terminal of the alternator, it will result in disconnection of all the electric loads from the alternator, thus leading to a great rise in output voltage from the alternator due to the load dump. Therefore, when the time when a voltage spike is expected not to occur is, like in the alternator taught in the above publication, waited for, it will result in an increased length of time for which an excessive voltage is appearing at the output terminal of the alternator. This may cause the voltage which exceeds the breakdown voltage to be applied to the MOS transistor or an electric power control circuit, so that it is damaged, thus resulting in a decrease in reliability in operation of the alternator.